From what i remember, 2x4870's more or less performed equivalently to a 5870 and the 6870 more or less performs equivalently (but a lil bit under) a 5870, so it should be 6870 = 2x4870's.
If ur thinking about upgrading it wouldn't really be necessary unless you wanted to play the couple of direct x 11 capable titles out there ;)

nah, im not just going to upgrade for dx11, it's going to require a boost in performance as well.

At this rate i won't need to upgrade until after i upgrade my CPU (and motherboard and ram)